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Clarification for only using feature IDs #40

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Nov 30, 2021
Merged

Clarification for only using feature IDs #40

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16a0863
Clarification for only using feature IDs
javagl Oct 27, 2021
60ea299
Elaborated external ID lookup
javagl Nov 16, 2021
a91e02a
Clarification for feature ID types in nodes and primitives
javagl Nov 23, 2021
07ffb50
Changed subsection title
javagl Nov 23, 2021
2318cb7
Changed second subsection title
javagl Nov 23, 2021
61fcc85
More details for externally used feature IDs
javagl Nov 23, 2021
20a81cc
Removed inlined example, only use the diagram
javagl Nov 23, 2021
ec52f7a
Removed duplication
javagl Nov 23, 2021
f7f34be
Removed blank lines
javagl Nov 23, 2021
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17 changes: 16 additions & 1 deletion extensions/2.0/Vendor/EXT_mesh_features/README.md
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Expand Up @@ -247,7 +247,22 @@ Each feature ID definition may include only a single source, so the following ar
- `featureId.offset` and `featureId.repeat` (for a Feature ID Attribute)
- `featureId.index` (for a Feature ID Texture)

Every `propertyTables` index must have an associated `featureIds` definition, but feature IDs may be defined without a property table. The `propertyTables` entry at index `i` corresponds to the `featureIds` entry at the same index. As a result, the length of the `featureIds` array must be greater than or equal to the length of the `propertyTables` array. Each (`featureId`, `propertyTable`) pair must be unique, but individual feature IDs and property tables may be repeated within a primitive or node.
Feature IDs may be assigned to primitives or nodes, but do not have to be associated with a property table. This is useful when the feature IDs are supposed to be resolved externally by an application.
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I think this could be worded a little clearly, and I think we can add another use case (using feature IDs for styling.)

Here's one possible wording:

Suggested change
Feature IDs may be assigned to primitives or nodes, but do not have to be associated with a property table. This is useful when the feature IDs are supposed to be resolved externally by an application.
Feature IDs may be assigned to primitives or nodes without being associated with a property table. This is useful when an application retrieves properties from an external resource (e.g. a database or REST API). This can also be used for identifying features in a mesh for styling in an application, even if no properties are stored.

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oh another thing, sets of feature IDs may be stored in primitives or nodes in the schema, but conceptually feature IDs themselves are only ever assigned to vertices, GPU instances or texels. it would be good to read through this section and make sure we're consistent with our wording.

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hm... in thinking about this some more, "sets" of feature IDs could get a little ambiguous, as all the FEATURE_ID_n semantics together could also be referred to as a "set of feature IDs"...

When in doubt, it might be better to be explicit and say "featureIds entries" or something like that?


> **Example:** A minimal example for assigning feature IDs to a mesh primitive, using an implicit vertex attribute: The default value for the `offset` is 0, and the value for `repeat` is 3. This means that the vertices will receive the feature IDs `[0, 0, 0, 1, 1, 1, 2, 2, 2, ...]`, causing the vertices of each triangle to receive the same feature ID.
>
> ```jsonc
> // Primitive:
> "extensions": {
> "EXT_mesh_features": {
> "featureIds": [
> { "repeat": 3 }
> ]
> }
> }
> ```

When feature IDs are associated with property tables, then every `propertyTables` index must have an associated `featureIds` definition. The `propertyTables` entry at index `i` corresponds to the `featureIds` entry at the same index. As a result, the length of the `featureIds` array must be greater than or equal to the length of the `propertyTables` array. Each (`featureId`, `propertyTable`) pair must be unique, but individual feature IDs and property tables may be repeated within a primitive or node.

Empty feature IDs (e.g. `{}`) are disallowed — a feature ID must explicitly set at least one property.

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